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Astron. Astrophys. 329, L45-L48 (1998)

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2. Observations and data reduction

The NGC 1068 speckle interferograms were obtained with the 6 m telescope at the Special Astrophysical Observatory (SAO) in Russia on October 3, 1996. The speckle interferograms were recorded with a 256 [FORMULA] 256 pixel NICMOS 3 camera through a standard K-band filter with center wavelength of 2191 nm and FWHM bandwidth of 411 nm. A typical speckle frame is shown in Fig. 1. The exposure time per frame was 200 ms, K-band seeing was about 1:005. 778 object speckle interferograms and 526 reference star (HIC 12014) interferograms were recorded. The scale was 30.82 mas/pixel and the field of view 7:009 [FORMULA] 7:009. Diffraction-limited images were reconstructed from the speckle data using the speckle masking method (Weigelt 1977; Lohmann, Weigelt, Wirnitzer 1983; Pehlemann, Hofmann, Weigelt 1992). The process includes the calculation of the average power spectrum and the average bispectrum and the subtraction of the detector noise terms from those. The 4-dimensional bispectrum of each frame consisted of [FORMULA] 49 million elements. No postprocessing by image restoration methods was applied to the speckle masking reconstructions. For the calibration of the flux the photometric standard star HIC 110609 ([FORMULA] BS 8541), chosen from Elias et al. (1982), was observed on September 30, 1996. We adopt the HIC 110609 flux to be 12.57 Jy at 2.2µm.

[FIGURE] Fig. 1. One of our 778 speckle interferograms of NGC 1068 taken through a K-band filter, demonstrating that it is possible to obtain speckles with high signal-to-noise ratio from a K [FORMULA] source, even under not very good seeing conditions. The observations were made using a NICMOS 3 array with 200 ms exposure time per frame and multiple read-out (4 [FORMULA]). Furthermore, the high-contrast speckles show that the NGC 1068 core is very compact since the speckles look like speckles of a point source. The shown field of view is 1:0085 [FORMULA] 1:0085.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997